Various automatic landing systems have become available during the past several years, often being incorporated in the aircraft autopilot or flight control system. Basically, these automatic landing systems are digital signal processors that execute control laws that cause the aircraft to follow glideslope and localizer signals so that the aircraft approaches the destination runway without manual control by the pilot. When the aircraft reaches a suitable point in its descent, the automatic landing system initiates a flare maneuver to arrest aircraft rate of descent (sink rate) so that the aircraft touches down on the runway at a proper position and at a suitable sink rate. The system then maintains the aircraft in a suitably straight path throughout the rollout landing phase by operating the aircraft control surfaces.
Although significant advances have been made in automatic landing system performance, the prior art has not provided a system that satisfactorily executes automatic landings when one of the aircraft engines has failed or has otherwise become inoperative prior to the time at which the landing is to be executed. Specifically, although some prior art systems may be capable of safely landing the aircraft with an engine inoperative, overall engine-out landing performance of such systems is such that the systems do not meet the all weather certification requirements of the aviation authorities (e.g., current Category III weather minimums). As is known in the art, these certification requirements impose limits or bounds on performance factors such as landing length, the point at which the aircraft must touch down after crossing runway threshold, the position of the aircraft relative to the runway center line at the point of touchdown and the amount of variation that is allowable relative to straight line travel during rollout.
To be of value in service and, as a practical matter, to qualify for certification, an automatic landing system must not only meet the formal certification requirements, but must also be acceptable to the vast majority of the pilots that fly the aircraft. In particular, unless the automatic landing system maneuvers the aircraft in a manner that substantially corresponds to standard manual flight practice, the pilot may become unduly concerned during an automatic landing procedure. Such concern may cause the pilot to assume manual control, even in situations in which the automatic landing is being made under low visibility conditions.
By way of example, some prior art systems that meet normal certification requirements (landings with all engines operative) exhibit an engine-out characteristic that is disfavored by pilots. In particular, if such an automatic landing system is engaged with one engine inoperative, the system places the aircraft in a "failed engine high" configuration. That is, the aircraft will roll slightly with the wing on which the operative engine or engines is/are mounted being lower than the wing on which the inoperative engine is located. This aircraft attitude is not generally acceptable to pilots in the event of cross wind conditions under which the wind direction is from the same side as the failed engine. Specifically, standard flight procedure for landings with an inoperative engine calls for the aircraft to be maneuvered and landed with the upwind wing low, thus maintaining maximum aircraft stability and control handling characteristics during touchdown and rollout.